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 Post subject: Re: KT120 Oddblocks
PostPosted: 23 Jul 2014, 01:52 
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gofar99 wrote:
The combined dissipation rating is 68 watts.


In other words, not a project for when it's 34C. outside and you don't have an A/C :hot: ;)

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 Post subject: Re: KT120 Oddblocks
PostPosted: 23 Jul 2014, 10:24 
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Joined: 04 Jun 2008, 20:59
Posts: 4205
Location: Arizona, USA
Hi, Not at 34C for sure. In the winter you can always "cuddle" with them (if you are careful not to touch the tubes). Now a quad of CX4000 ....that would heat the house. The power consumption of the amps is not quite as bad as I originally expected. If I believe my power meter (I do) then they use between 215 and 225 watts each depending a bit on the AC mains voltage. I figure an overall efficiency of about 20% isn't horrible for a class A power amp.

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 28 Jul 2014, 23:53 
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Joined: 08 Sep 2013, 18:43
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OK, Digging a little deeper.

I'm considering using 2 x AS4T-360 toroidals from Antek
These have 2 x 360 windings each at 550ma and 2 x 6.3V windings each at 4 amps
Primaries (series-ed up) take 230VAC in Australia we run 240VAC
For a 300ma draw, the Power Supply will drop ~ 15VDC via the 2 x 100R paralleled resistors and we'll get a further drop around 40VDC between the cathode and ground from the CCS.
The Antek transformer regulation is not published, I'd expect a 10% drop or so, but otherwise I'm comfortable with the B+.
If it runs a tad high I can tweak it with the power supply resistors.

Re heaters,
I'd have dual 6.3VAC windings connected in series and the KT120 heaters would also be connected in series as per Bruce's PS schematic.
As a sanity check (in case I'm having a blond moment) the total heater current drawn for the 2 x KT120s in this configuration would be 2 Amps.
Can someone confirm this please ?
Eitherway the 6.3 @ 4amp windings on the Antek should cope though.

BTW Has anyone measured the actual KT120 heater current, the stated value in the datasheet is not as high as my gut feel expects?

Of course I'll need a dropping resistor for the rectified (but unregulated KT heater supply 's), no drama, I figure I'll need ~2.8 ohms and so what if I add another 11 watts of dissipation to the mix.

Another question re the Power Supply schematic,
The ground of the HT circuit has a 120 ohm resistor paralleled to a 0.1uf cap and this is shown tied back to the Line filter.

I'm guessing we are keeping the mains ground and Audio grounds separated by 120 ohms (but coupling them for RF).
I'm not 100% sure, can someone please explain what the intention is here ?
It also raises the question, are the HT bridge rectifier and amplifier grounds otherwise isolated from the chassis ?

Cheers


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 Post subject: Re: KT120 Oddblocks
PostPosted: 29 Jul 2014, 10:00 
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Location: Arizona, USA
Hi That ought to work. I have measured the heater current and it does seem to vary a bit from tube lot to tube lot. Within any one bunch it is however consistent. The lowest I measured was 1.95 amps the highest was 2.3. So plan on 2.3 and you will be safe.

The separation of grounds is for a number of reasons. Rather than go over it again I recommend you look at my write up in the tech section of the site. You are though correct that the chassis (earth) ground is separated from the signal and power supply grounds. The PS and signal must eventually come together and I almost always do that at the input for the signal source.

BTW I have no issues with Antek trannies, they just don't make enough different types to suit my needs and it is not easy to get them to custom wind any.

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 28 Aug 2014, 20:37 
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Hi Everyone, Fame has to some extent reached the KT120 amps. The assembled version (VC Audio MB45) was just reviewed on Analog Planet. All you happy builders now have some proof that simple designs can really sound nice.

http://www.analogplanet.com/content/vc- ... lifier-kit


Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 28 Aug 2014, 22:00 
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Joined: 08 Aug 2009, 03:11
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gofar99 wrote:
...and it is not easy to get them to custom wind any.


Oh, he listens quite readily when you mention a pallet load ;)

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* Ratings are for transistors - tubes have guidelines*
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 Post subject: Re: KT120 Oddblocks
PostPosted: 29 Aug 2014, 07:38 
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Joined: 05 Jun 2012, 13:10
Posts: 196
Location: Darboy, WI, USA
Bruce, Congrats on the review and thanks for sharing.

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 Post subject: Re: KT120 Oddblocks
PostPosted: 07 Sep 2014, 15:33 
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Joined: 14 Feb 2010, 13:13
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Location: Lisbon, Portugal
I´ve reading this thread for a while, but i´m still confused...

What is the ideal audio-transformer impedance?

There are a lot of offers at Edcor, and I would like to have the right ones.


Cheers,

Miguel


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 Post subject: Re: KT120 Oddblocks
PostPosted: 07 Sep 2014, 15:56 
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Location: Arizona, USA
Hi The ones I recommend are the CXPP70-MS-3.5K. They are BTW used in the kits and built amps. You need the 70 watt size to handle the large amount of DC idle current (nearly 300ma). It is the smallest one in the Edcor line that can handle that. The 3.5K matches well with the class A operation. If you were running AB (not possible in this design) then a lower value of both wattage and impedance would be more appropriate.

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 07 Sep 2014, 16:57 
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Thank you Bruce! :D


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